1. Field of Invention
The present invention relates to flexible wiring substrates applied to COF (Chip On Flexible) modules, in which a plurality of semiconductor chips are mounted on a substrate composed of a flexible insulating thin-film provided with a conductive pattern thereon. More particularly, the present invention relates to a high density mountable flexible wiring substrate in which the conductive patterns are provided on both surfaces thereof.
2. Description of Related Art
Flexible wiring substrates have advantages in flexibility and modifiable shape thereof over rigid wiring substrates. Accordingly, high density mounting of ICs and decrease in module sizes are advantageously performed. In particular, flexible wiring substrates are essential for miniaturizing various mobile devices.
Many flexible substrates have conductive patterns printed on both surfaces thereof, and connection structures by through holes are typically formed. That is, penetrating holes are formed on through hole lands by using a drill or the like, and through-hole plating (copper plating) or the like is performed. As a result, predetermined conductive patterns on both surfaces of the substrate are connected with each other.
However, in conventional flexible substrates, when through-hole plating is performed, since there is a step (button plating) of plating after removing a resist at which through hole lands are formed, the number of manufacturing steps are increased, and as a result, the manufacturing cost is increased. In contrast, when plating is performed without performing resist coating, conductive pattern widths on the substrate are increased, and as a result, patterns having fine pitches are difficult to form.
The present invention was made in consideration of at least the situations described above, and an object of the present invention is to at least provide a flexible wiring substrate which can be applied to conductive patterns having fine pitches, in which reliable connection between through holes can be realized at a lower cost by a smaller number of manufacturing steps.
A flexible wiring substrate according to one exemplary embodiment of the present invention may consist of a flexible substrate having a first conductive pattern provided with not more than a first pitch formed on a main surface of the flexible substrate and a second conductive pattern provided with not less than the first pitch formed on a rear surface of the flexible substrate, and a via connection structure in which a first conductive material printed from the main surface side and a second conductive material printed from the rear surface side contact each other in a penetrating hole formed in the substrate so as to be enclosed by respective parts of areas of the first and the second conductive patterns, wherein, compared to the first conductive material, the second conductive material is composed of a material having a lower viscosity.
According to the flexible wiring substrate of this exemplary embodiment of the present invention, by using a flexible thin substrate, a via connection structure can be formed by printing performed on both sides of the substrate. Conductive patterns formed on the main surface and the rear surface of the substrate have different pitches from each other. In order to realize a reliable via connection structure which can be applied to the substrate having the conductive patterns described above, the structure is formed of conductive materials having viscosities differing from each other.
A flexible wiring substrate according to another exemplary embodiment of the present invention may consist of a flexible substrate having a first conductive pattern provided with not more than a first pitch formed on a main surface of the flexible substrate and a second conductive pattern provided with not less than the first pitch formed on a rear surface of the flexible substrate, and a via connection structure in which a first conductive material printed from the main surface side and a second conductive material printed from the rear surface side contact each other in a penetrating hole formed in the substrate so as to be enclosed by respective parts of areas of the first and the second conductive patterns, wherein the first conductive material having a first diameter extends to the part of the area of the first conductive pattern, and the second conductive material having a second diameter larger than the first diameter extends to the part of the area of the second conductive pattern.
According to the flexible wiring substrate of this exemplary embodiment of the present invention, by using a flexible thin substrate, a via connection structure can be formed by printing performed on both sides of the substrate. Conductive patterns formed on the main surface and the rear surface of the substrate have different pitches from each other, and in order to realize a reliable via connection structure which can be applied to the substrate having the conductive patterns described above, the structure is formed of conductive materials having diameters differing from each other.
A flexible wiring substrate according to another exemplary embodiment of the present invention may consist of a flexible substrate having a first conductive pattern provided with not more than a first pitch formed on a main surface of the flexible substrate and a second conductive pattern provided with not less than the first pitch formed on the rear surface of the flexible substrate, and a via connection structure in which a first conductive material printed from the main surface side and a second conductive material printed from the rear surface side contact each other in a penetrating hole formed in the substrate so as to be enclosed by respective parts of areas of the first and the second conductive patterns, wherein the second conductive material forms the second conductive pattern.
According to the flexible wiring substrate of the present invention, by using a flexible thin substrate, a via connection structure can be formed by printing performed on both sides of the substrate. In the case in which conductive patterns formed on the main surface and on the rear surface of the substrate have pitches different from each other, when the conductive pattern is formed on the rear surface having relatively no fine pitches thereon, a reliable via connection structure can be simultaneously realized.